In its initial stages of infancy the telecommunications industry was entirely closed. Every manufacturer had its own proprietary set of communication standards. A subsequent push for open standards by industry leaders led to an explosion of new, open standards that define everything from telecommunication operating systems and digitally encoded voice streams, to IP telephony (i.e., phone and facsimile over the Internet). Among the industry's open standards is the Open Systems Interconnection (OSI) model developed by the International Standard Organization. The OSI model is the only internationally accepted framework for telecommunication standards.
The OSI model organizes the communications process into seven different categories and places these categories into a layered sequence based on their relation the user. Generally, layers 7 through 4 deal with end to end communications between a message source and a message destination, while layers 3 through 1 deal with network access such as data transmission and routing. Of particular interest here are layers 3 and 2, the Network layer and the Data Link layer, respectively.
Each layer in the OSI model operates under a different protocol (i.e., a set of rules governing the format for the exchange of messages). In most cases, the Network layer operates under what is called the Internet Protocol (IP), which provides end to end communication between host machines that are uniquely distinguished by their individual IP addresses. In other words, each host machine, server, or router within an IP network is IP capable, and individually identifiable by its own IP address.
The fundamental unit of information passed across the Internet in an IP communication is an IP datagram. The IP datagram contains the source and the destination address along with the data and a number of fields, which define the length of the datagram, the header checksum, and the flags that indicate whether the datagram has been fragmented.
The Data Link layer operates under any number of protocols including, but not limited to, Asynchronous Transfer Mode (ATM), Frame Relay, and Ethernet. A link layer can be thought of as providing a transport service to its upper layers across sub-networks or subnets (i.e., Wide Area Networks (WANs) or Local Area Networks (LANs)). An IP datagram may traverse several subnets from an IP source to its IP destination. A Data Link layer protocol provides a delivery service to the Network layer within the context of a given subnet.
Over the past several years, the demand for telecommunication services such as broadband access (i.e., through cable modems and digital subscriber lines (DSL)) and virtual private inter-connect services (i.e., through an Asynchronous Transfer Mode (ATM) Cell Relay, or a Frame Relay network), has continued to grow with an emerging acceleration for broadband IP services (e.g., IP Virtual Private Network (IP-VPN)). As service providers rollout these services, they have encountered a demand for performance guarantees of those services, particularly for business customers that rely on mission critical and distributed applications. To meet these demands, many service providers now provide operational and performance guarantees for specific metrics such as: network uptime, mean time to repair (MTTR), average packet latency, and bounded packet loss ratios. These parameter values for metrics are typically specified and guaranteed in the form of a Service Level Agreement (SLA). The SLA is in effect, a contract between the service provider and the customer, where the provider agrees to meet a given set of performance objectives. The SLA generally provides detailed language of what the performance objectives are and what credit the customer can receive if the provider network does not meet those objectives.
Broadband service is generally provided from an IP backbone through several subnets to a broadband access device that is located at or near the customer premise. The broadband access device is generally connected to a customer provided router or modem, each of which is located within the customer premise. Because the service provider does not have control over customer provided equipment (CPE), the scope of the SLA often terminates at the broadband access device (i.e., the demarcation point) and does not extend into the CPE domain.
Some key metrics for the SLA's can be measured using what is known in the IP Network as a “ping” command. The ping command sends a “signal”, which is generally referred to as an Internet Control Message Protocol (ICMP) Echo Request Packet, to a destination IP address and waits for an ICMP Echo Reply. This reply, or lack thereof, may be used to determine and quantify some of the metrics for the SLA. The problem, however, is that the ping command only works on IP capable devices that have an IP address. Therefore, it is not possible with the existing OSI configuration for a service provider to “ping” from a provider server or router directly to the broadband access device, because broadband access devices operate under Data Link layer (layer 2) protocols and are generally not IP capable devices. Even in cases where the broadband access devices and CPE routers are integrated, fire-walling functions may prohibit such devices from responding to ping requests from external and/or unknown IP devices. Similarly, it is not possible for a service provider to “ping” from an IP host or router to layer 2 network elements within a particular subnet, even though such network elements may support the analogous OAM function (such as an OAM loopback).
Analogous to a “ping” command, the Data Link layer generally employs what is referred to as an Operations, Administration, and Maintenance (OAM) loopback function, which provides network fault indication, performance information, and diagnostic functions within the Data Link layer. The communications standard implemented in the Data Link layer depends on the link layer protocol. For example, many broadband access networks implement Asynchronous Transfer Mode (ATM), however, other protocols including, but not limited to, Frame Relay, Ethernet, or Multiple Protocol Layer Switch (MPLS) may be used.
Because each OSI model layer implements a different communications protocol, testing and operations management functions like the “ping” command and the OAM loopback function are unrecognizable by devices in differing layers. In fact, hosts and routers in the IP Network layer are generally unaware of Data Link layer elements such as switches and multiplexers. Accordingly, there is a need for a system and method that is capable of cross layer communication in an OSI model network such that testing and operation commands such as the “ping” mechanism are recognizable to elements in other layers.